PEP 12: Allow "Pending" as Discussions-To

[hugovk]

There's often confusion about whether the discussion thread should be created before or after the draft PEP has been merged.

It's often better to merge the PEP first, and the the thread can use the https://peps.python.org/ URL instead of disseminating a temporary RTD URL that will soon be outdated and later deleted.

The linter is confusing too: "Discussions-To must be a valid thread URL or mailing list" makes some think the thread must be created first to pass, it's not obvious they can omit the field to satisfy the linter.

Finally, PEP 12 says "it is okay to just list the venue name initially" but the linter requires a thread or ML URL.

Instead, I think it may improve things if authors can write "Discussions-To: TBA" "Discussions-To: Pending" without the linter complaining. We can then merge the draft PR, they can open the thread, and then replace the TBA Pending.

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[Rosuav]

👍 Definitely in favour.

[warsaw]

Huge +1, as there's always the "publish PEP, start the DPO thread, update the PEP dance"

That said, I'd bikeshed against using an initialism, and just go with "Pending"

[brettcannon]

I went ahead and merged as Barry and I were +1 and everyone overall seemed positive on the idea.